The word "passivation" is commonly used in the semiconductor art to refer to surface treatments and/or surface layers provided above the intersection of a PN junction with a semiconductor surface to promote junction stability. It is also well known in the art to form diodes having passivated junctions, including Zener diodes.
There are many different procedures used for forming PN junctions. For example, it has been customary to form high voltage Zener diodes by diffusing impurities into comparatively lightly doped substrates from liquid or gaseous dopant sources. Long drive-in times are frequently employed to produce deep junction and thermal oxides are employed for junction passivation. While such methods are satisfactory for a wide range of Zener devices, they have significant deficiencies that are exacerbated when there is a need for higher voltage devices, e.g., devices having V.sub.z .gtoreq.about 200-400 volts.
Among these deficiencies is the need to use comparatively deep diffusions, often resulting in very long drive-in times at high temperatures (e.g. 60 hours at 1250.degree. C.). This increases manufacturing cost due to low equipment utilization and yield loss due to defect formation.
A further deficiency is the need to thin the comparatively lightly doped substrates after diffusion, passivation and front-side metallization and before back-side metallization, in order to reduce the series resistance of the devices. Typically, a 100 mm diameter semiconductor wafer is lapped or polished to reduce its thickness from .about.0.5 mm to .about.0.2 mm. Thinning and post-thinning processing steps are well known sources of wafer breakage and yield loss during semiconductor device manufacture because of the fragile nature of the thinned wafers.
A still further deficiency observed with prior art devices, is their comparatively poor electrical stability when subjected to high temperature reverse bias (HTRB) at, for example T.gtoreq.150.degree. C. It has been observed that many commercially available Zener devices fail when subjected to 150.degree. C. HTRB, despite the fact that the different devices employ different passivation arrangements which give satisfactory performance under more benign test or operating conditions.
Thus, a need continues to exist for diodes or other passivated PN junctions, including Zener diodes, that are easier and cheaper to manufacture and that exhibit improved stability under HTRB tests, especially for T.gtoreq.150.degree. C.